1. Field of the Invention
The present invention relates to a fabrication method of a semiconductor device, and particularly to a fabrication method for forming plug electrodes as wiring between MOS field effect transistor (MOSFET) elements having gates, sources, and drains that are converted to a silicide with self alignment. The present invention further relates to a transistor fabrication method in which, when forming a diffusion barrier layer of a plug electrode material, which is wiring between MOS field effect transistor (MOSFET) elements, by a high-temperature sputtering method, the adhesion between a metal silicide film and the diffusion barrier layer of the plug electrode material is improved, and a diffusion barrier layer of a plug electrode material having highly uniform low resistance is formed.
2. Description of the Related Art
One method of fabricating a semiconductor device consists in forming a plug electrode as the wiring between MOS field effect transistor (MOSFET) elements. The procedures of this fabrication method are explained with reference to the sectional views of FIGS. 1A to 1F.
First, as shown in FIG. 1A, n-well 202 is formed in silicon substrate 201 by a known method. Field oxide film 203 is next formed by a selective oxidation method. Gate insulation film 204, for example, a silicon oxide film are made up by turn, and a polycrystalline silicon are successively grown on the active region that is surrounded by this field oxide film 203, and the polycrystalline silicon is doped with phosphorus by a known method to decrease the electrical resistance of the polycrystalline silicon.
The polycrystalline silicon is next patterned by the known methods of photolithography and dry etching to form gate electrode 205, as shown in FIG. 1A. Next, a low-concentration n-type impurity layer (not shown) and a low-concentration p-type impurity layer (not shown) are formed by photolithography and ion implantation. Side wall spacers 206 composed of a silicon oxide film or a silicon nitride film are next formed on the sides of gate electrode 205 using a known CVD method and an etching method.
A source-drain region of an n-type impurity layer and a source-drain region of a p-type impurity layer are next formed by photolithography and ion implantation. By these processes, n-type source-drain region 207 and p-type source-drain region 208 are formed as an LDD (lightly doped drain )structure.
Next, after removing the native oxide film (not shown) on the silicon substrate surface and on the surface of the polycrystalline silicon, which is the gate electrode, cobalt silicide film 209 is formed with self-alignment only over the gate electrode and diffusion layer according to a known method, as shown in FIG. 1B.
An insulating film such as a silicon oxide film is next grown using a known CVD (chemical vapor deposition) method, and the insulating film surface is then leveled using a known CMP (chemical mechanical polishing) method. The insulating film is next patterned using known photolithography and dry etching methods, and plug hole 210 is then formed for forming a plug electrode, as shown in FIG. 1C.
After cleaning the silicon substrate with, for example, a mixed solution of dilute sulfuric acid and hydrogen peroxide, the native oxide film (not shown in the figure) that has formed on the surface of the silicon substrate is removed by a known RF (radio frequency) sputtering method in an RF sputtering chamber which is attached to a sputtering device using plasma in which oxygen is mixed. At this time, a portion of cobalt silicide film 209 that is present at the bottom of plug hole 210 is etched as shown in FIG. 1D.
Titanium film 211 is next grown as shown in FIG. 1E by a sputtering method in an argon plasma atmosphere and under a heating condition of 300xc2x0 C. in a chamber of a sputtering device other than the above-described RF sputtering chamber. Still further, titanium is sputtered using a known reactive sputtering method under a heat condition of at least 300xc2x0 C. in a mixed argon-nitrogen gas plasma atmosphere to form titanium nitride film 212 on the silicon substrate. A Rapid Thermal Annealing process (RTA) is then carried out to form titanium silicide film 213 at the interface of titanium film 211 and cobalt silicide film 209.
After cleaning the silicon substrate with, for example, an aqueous mixed solution of dilute sulfuric acid and hydrogen peroxide, a tungsten film is grown using a known CVD method, following which the surface of the tungsten film is leveled using a known CMP method to form plug electrode 214 as shown in FIG. 1F.
The above-described method of fabricating a semiconductor device was employed to convert the polycrystalline silicon of a p-type electrode that had been doped with boron ions at a high concentration of 3xc3x971015 atoms/cm2 to cobalt silicide and then form a plug electrode above it. Subsequent measurement of the sheet resistance of the plug portion showed that the sheet resistance was at least 11 xcexa9/(unit area), and a comparison of the sheet resistance within the silicon substrate surface revealed a variation of at least 3 xcexa9. Similarly, a p-type diffusion layer electrode was converted to cobalt silicide and a plug electrode then formed above it. Subsequent measurement of the sheet resistance of the plug portion showed a sheet resistance of at least 11 xcexa9/(unit area), while a comparison of the sheet resistance within the silicon substrate surface revealed a variation of at least 6 xcexa9/(unit area).
The above-described method of fabricating a semiconductor device was also employed to convert the polycrystalline silicon of an n-type gate electrode on the silicon substrate that was doped with arsenic ions at a high concentration of 5xc3x971015 atoms/cm2 to cobalt silicide and then form a plug electrode above it. Subsequent measurement of the sheet resistance of the plug portion showed that the sheet resistance was at least 10 xcexa9/(unit area), and a comparison of the sheet resistance within the silicon substrate surface revealed a variation of at least 4 xcexa9/(unit area). Similarly, when a p-type diffusion layer electrode was converted to cobalt silicide and a plug electrode formed above it, subsequent measurement of the sheet resistance of the plug portion showed a sheet resistance of at least 10 xcexa9/(unit area), and a comparison of the sheet resistance within the silicon substrate revealed a variation of at least 4 xcexa9/(unit area).
In the above-described fabrication method of a semiconductor device, the sheet resistance of a plug electrode was high and the variation within the surface was large. It is therefore an object of the present invention to provide a fabrication method of a semiconductor device for forming on a silicide film a refractory metal film having lower resistance than the prior art.
As the result of continued research to realize a fabrication method of a semiconductor device that can achieve the above-described object of the present invention, the inventors of the present invention discovered that the increased sheet resistance of the plug portion and the large variation of sheet resistance within the surface both result from etching a portion of the metal silicide film by the RF sputtering method that is carried out immediately before growing the titanium film that is to serve as the diffusion barrier layer of the plug electrode.
As a result of further research, the inventors of the present invention also discovered that, in a case in which the titanium film is grown without carrying out the above-described RF sputtering method before depositing the titanium film that serves as the diffusion barrier layer of the plug electrode, a titanium silicide film is not formed at the interface of the titanium film and metal silicide film. As a result, the sheet resistance of the plug portion is further increased, variation within the surface becomes larger, and adhesion between the titanium film and metal silicide film is adversely affected, in some cases giving rise to exfoliation of the film.
According to the present invention, a method of fabricating a semiconductor device is provided in which, in a method of fabricating a semiconductor device in which a metal film, which is to serve as a diffusion barrier layer of an inter-element plug electrode of semiconductor elements, is formed over the entire surface of a silicon substrate using a sputter deposition method; wherein before the sputter deposition of the metal film, all portions of the substrate are covered by a photoresist except for portions of plug formation, following which silicon ions are implanted into the substrate, the photoresist is removed, and finally, the substrate is cleaned with a chemical fluid.
The present invention consists in implanting silicon ions in the bottom portion of a plug hole and then depositing a titanium film, which is to serve as the diffusion barrier layer of the plug electrode, whereby a titanium silicide film of low resistance is formed at the bottom of the plug hole without lowering the resistance of the plug electrode and without bringing about peeling of the film.
As for an explanation of this effect, silicon ions are implanted into the silicon substrate after etching the plug hole with the photoresist still in place. The photoresist is next removed and the silicon substrate is cleaned with a chemical fluid and then conveyed to a sputtering device. After growing a titanium film and titanium nitride film by a sputtering method, annealing is performed to form the plug electrode. The sheet resistance of the p-type gate electrode is compared for cases in which the surface etching process by a RF sputtering method is carried out or omitted immediately preceding the above-described titanium sputtering process.
Compared to a case in which surface etching is not performed, the sheet resistance for a case in which surface etching is performed by the RF sputtering method is approximately 10% higher, and the variation in sheet resistance within the surface is approximately 10% larger.
RF etching reduces the film thickness of the cobalt disilicide (CoSi2) film at the bottom of plug holes. Variation in plasma concentration within the silicon wafer surface is considerable when RF etching is carried out, and the thickness of the cobalt disilicide (CoSi2) film at the bottom of plug holes therefore also varies considerably within the wafer surface. As a result, the sheet resistance of the silicide film as a whole becomes high and variation in sheet resistance increases in cases in which the area of the open hole of a plug hole is large. As a result, resistance of the plug electrode increases and variation in resistance also becomes large.
If RF etching is not carried out, however, the thickness of the cobalt disilicide (CoSi2) film does not decrease and the sheet resistance of the silicide film does not become high.
To continue, a plug hole is etched, the photoresist is removed, the silicon substrate is cleaned with a chemical fluid, and the silicon substrate is conveyed to a sputtering device. The titanium film and titanium nitride film are next grown by a sputtering method, annealing is performed, and the plug electrode film is grown to form the electrode.
The shape of the silicon wafer is compared for a case in which the surface etching process is performed by an RF sputtering method immediately before the above-described titanium sputtering process, and for a case in which the surface etching process is not carried out.
In the case in which RF etching is carried out, despite long time heating of the wafer to grow a tungsten film by a CVD method for the purpose of forming a plug electrode, film exfoliation on the silicon substrate surface cannot be observed by the unaided eye and a good shape is maintained. In a case in which RF etching is not carried out, however, film peeling could be observed by the unaided eye in portions of the silicon wafer immediately following the above-described growth of tungsten. Even in cases in which film peeling did not occur, the resistance of the plug electrode in some cases increased markedly (for example, eight times the normal value) in portions of the silicon wafer.
Strong bonding is maintained between cobalt and silicon atoms in the cobalt disilicide (CoSi2) film at the bottom of a plug when RF etching is not carried out. When titanium and silicon react to form titanium silicide, however, the silicon atoms are reaction diffusion xe2x80x9cseeds.xe2x80x9d
Reaction between titanium and silicon therefore tends not to occur when RF etching is not carried out, and a titanium silicide film is therefore difficult to be formed.
Furthermore, cobalt that has undergone conversion to a silicide has a characteristic that impedes formation of an alloy with titanium.
The significance of the foregoing explanation is that little chemical bonding occurs between a cobalt disilicide (CoSi2) film and a titanium film, and adhesion between the two films is therefore extremely poor, and when a titanium film expands during the course of long-term heating, the titanium film therefore tends to peel away from the cobalt disilicide (CoSi2) film. Since a large force is also exerted on the overlying tungsten film, exfoliation of the entire structure tends to occur.
Exfoliation similarly occurs if RF etching is not carried out even in cases in which a titanium nitride film is grown directly on the cobalt disilicide (CoSi2) film. This state arises from the fact that the cobalt and silicon that form the cobalt disilicide (CoSi2) film tend to react with neither the titanium nor the nitride in the titanium nitride film.
In contrast, a portion of the bonds between cobalt and silicon that are present in the cobalt disilicide (CoSi2) film at the bottom of a plug are cut in the plasma atmosphere when RF etching is carried out, and titanium atoms and silicon atoms therefore tend to react relatively easily in the annealing that immediately follows sputtering, whereby a titanium silicide film having high adhesion with the cobalt disilicide (CoSi2) film, is formed. Adhesion between a titanium silicide film and a titanium nitride film is high, and exfoliation therefore does not occur.
In a case in which silicon ions are implanted immediately following hole formation of a plug hole by etching, exfoliation does not occur even if RF etching is not carried out. This state occurs because the cobalt disilicide (CoSi2) film at the bottom of a plug hole becomes non-crystalline due to the ion implantation, and a portion of the bonds between cobalt and silicon that are present in the film are cut, whereby titanium atoms react relatively easily with silicon atoms in the process of annealing that immediately follows sputtering, and a titanium silicide film having high adhesion with cobalt disilicide (CoSi2) film is formed. In addition, the amorphous cobalt disilicide (CoSi2) film is recrystallized during the annealing that immediately follows sputtering. Exfoliation therefore does not occur and resistance does not increase.